Composite Steel Beam (Partial Length Only)

[Lion06]

Hello All -

I have an interesting situation in which a new opening needs to be added in an existing floor. The opening will remove the slab from both sides of the beam for the 2' length at one end. Normally, I would check this as non-composite (which was done) and reinforce it, if necessary. Reinforcing this beam, however, is difficult (access issues) and I'm trying to determine if you can use composite behavior for part of the length of the beam. I can't find anything in AISC regarding this. Does anyone know of a reference document for this condition.

Thanks!

 

[JoshPlumSE]

What's the total length of the beam? And, it's only the end 2 ft that don't have composite deck on it?

If the beam is 20 ft or more in length, I'd be fine with considering it composite still. Below is an excerpt from the documentation of a structural software program I worked on year ago. I wrote up the documentation, but I didn't come up with the rule. End Off Set Example 1 is very similar to the situation you describe where there is an opening at the very end of the beam and the user wants to exclude consideration of this from the calculation of "effective width" of the composite beam.

That being said, I would still feel better if the "effective width" were reduced to something more like an "average effective width". Let's say the effective width for the other 18 ft of the beam is 8 ft (4 ft on each side), then I'd use an effective width of something 6 ft instead of the 8 ft. And, I'd be very careful that I have enough studs on this side of the beam to develop the max moment.

 

[KootK]

1) It's good to see you back here.

2 I've got nothing for formal documentation of a method.

3) Other than the effort involved in executing it, I'd have no qualms about treating it as a two-part beam design:

a) Part 1 = the bare steel beam at the end, with a moment at the interface.

b) Part 2 = the composite steel beam remaining, with a moment at the interface.

c) Hopefully get by on stud capacity without relying on the studs landing between the openings although one might be able to argue that even those are still effective for aggregate horizontal shear capacity.

d) Effective width load spread starts at the interface.

 

[Lion06]

Josh - It's about 22' long and the last 2'-9' +/- of slab will be removed.

I've had the condition you describe above come up and I generally check that as composite with slab on one side only (which is generally pretty conservative). With losing the slab on both sides for the last 2'-9", I have a couple concerns. First is getting enough studs in the shortened zone between the new slab cut and the midspan of the beam. Second is the potential for an increase in the demand on the end studs near the opening.

 

[Lion06]

Thanks Koot. It's been a busy 10 years resulting from upheavals in my personal life.

Good to know it doesn't seem unreasonable to look at it with part composite and part not.

I was thinking it's similar to a beam in which you reinforce it with a steel plate or WT for only part of the length. The steel reinforcement requires extending the reinforcement (in this case, the composite slab) beyond the point where it's needed by an anchorage length. I'm unsure how to account for that aspect of it.

 

[Agent666]

First is getting enough studs in the shortened zone between the new slab cut and the midspan of the beam

This assumes max moment is at midspan. If it isn't (often it isn't for beams supporting other beams), just pointing out that needs additional consideration if applicable.

 

[Agent666]

The composite action only kicks in once you have the minimum number of studs dictated by your code for the shear span. That is kind of the development concept you're referring to. I can't say how aisc treats this specifically. But often codes have some minimum like you need at least 5 rows of studs before you can get the composite action starting.

Edit - but provided you have the required minimum number of studs, then each stud contributes its capacity to the composite action. So it starts at edge of opening. It's no difference to the studs at end of simple supported beam. You're not required to discount any of them for example. Similar scenario in your case.

 

[KootK]

The steel reinforcement requires extending the reinforcement (in this case, the composite slab) beyond the point where it's needed by an anchorage length.

Yeahhhh... there's that. However, I am of the opinion that composite steel beams are not entirely kosher mechanically to begin with. Check out this thread of mine from 2014 which may have a ring of familiarity to it: Link. Long story short, I don't feel that north American detailed composite beams are entirely legit with respect to strict mechanics of materials theory even when you don't have the missing bits of slab. So I'm inclined to not sweat it too much when the openings are there. I shall hang my hat on composite beam design having a semi-empirical character to it.

Some non-quantifiables that may be working in your favor here:

1) As I mentioned, if you've still got concrete and some edge distance on the studs between the openings, they may still be doing something.

2) If this beam has similar neighbors, it may actually be able to utilize the studs on its neighbors to a degree. You'll not hear of it in the AISC manual or in our software manuals but, all said and done, these things are systems, not just discrete elements. This kind of alters the calculus on a %-difference analysis of the situation.

 

[BridgeSmith]

The capacity of the composite section may be limited by the shear capacity of the studs between the point under consideration for moment capacity and the non composite section. So, the moment capacity will increase as you move farther into the composite section and there are more studs to transfer the compression force in the slab to the steel beam. If you want to know the capacity of the composite beam at any point, add up the shear capacity of the studs between that point and the non composite section, and use a cross-sectional area of concrete with that compression capacity as the effective flange. If the studs are equally spaced, the effective flange will vary linearly from zero at the last shear studs to the point where the sum of the shear capacities of the studs equals the compression capacity of the full effective flange, or the tension capacity of the entire steel beam, whichever is smaller.

Rod Smith, P.E., The artist formerly known as HotRod10